Measuring probe

ABSTRACT

A measuring probe comprises a stylus-carrying member on which a stylus is magnetically retained. The carrying member is supported on a flat diaphragm spring for tilting movement relative to an intermediate member. The intermediate member is supported on two further diaphragms for linear movement relative to a housing. The spring rates of diaphragm and further diaphragms are chosen to give the probe the desired relative force/displacement characteristics in X,Y and Z.

BACKGROUND TO THE INVENTION

1. Field of the Invention

The present invention relates to a measuring probe, and relates inparticular to a probe having the capability of measuring the deflectionof a stylus assembly from a zero or rest position defined by a resilientplanar support. Probes of this type may be used for scanning the surfaceof a workpiece to determine its shape.

2. Description of Related Art

Such a probe is known, for example, from UK Patent Specification No.1,573,447. This specification describes a probe in which a stylusassembly, which has a work-contacting stylus attached thereto, issupported in the rest or zero position (i.e. the position in which noexternal load is applied to the stylus) by a planar spring. Probes ofthis type have few moving parts and therefore have the advantage ofbeing relatively cheap to manufacture.

However, these probes suffer from a problem in that the force requiredto produce a given deflection of the stylus from the rest position inthe direction of its longitudinal axis, (hereinafter referred to as theZ axis of the probe), and the force required to produce the samemagnitude of deflection in the sense of tilting the stylus relative tothe Z axis are interdependent. That is to say, that any modification tothe probe which alters the spring rate in respect of one of thesemotions, will also affect the rate in respect of the other.

For example, in the above described probe design, it is usually the casethat the spring rate in the Z direction is greater than the spring ratein the sense of tilting of the stylus.

This gives rise to a problem when the probe is used as an analogue probefor scanning a workpiece surface, where the stylus is in continuouscontact therewith, in that, when the stylus tip meets a change in theinclination of the surface which increases the pressure on the stylus inthe Z axis, the stylus may slide sideways on the surface due to thelesser force restraining tilting thereof instead of remaining in thedesired plane.

In many scanning operations the machine control system works on thebasis of calculating from the probe outputs of X, Y and Z deflections,the vector which is normal to the surface, and then driving the probe ina direction normal to the calculated vector. Thus, where the probereadings are giving a false indication of the surface inclination, (dueto sideways sliding of the stylus) the control will drive the probe inthe wrong direction and this can slow down the scanning process.

SUMMARY OF THE INVENTION

The present invention seeks to ameliorate these problems by providing ameasuring probe of the above mentioned type in which these two forcesmay be adjusted independently.

According to the present invention a measuring probe comprises:

a fixed structure by which the probe may be connected to a movable armof the machine;

an intermediate member;

supporting means for supporting the intermediate member for linearmotion along an axis;

means for applying a biasing force to said intermediate member when saidintermediate member is displaced from a rest position along said axis,said biasing force acting to return said intermediate member to saidrest position;

a diaphragm provided on said intermediate member, and extending in aplane orthogonal to said axis; and

a stylus carrying member for supporting a workpiece contacting stylusconnected to said diaphragm to enable tilting movement of said carryingmember, and thus of said stylus relative to said axis.

Preferably, the supporting means are provided by a pair of furtherdiaphragms whose combined spring rate in the direction of said axis may,for example, be chosen so that it is equal, for a predetermined lengthof stylus, to the force-displacement characteristic in respect oftilting of said stylus-carrying member (and thus said stylus). The forcerequired to produce a given magnitude of deflection in the sense oftilting of the stylus and in the sense of longitudinal deflectionthereof will thus be the same.

An independent aspect of the present invention relates to a transducerarrangement which may be used in conjunction with the probe describedabove. According to a second aspect of the present invention there isprovided a transducer assembly comprising: a light source provided onthe fixed structure for generating a beam of light; a photodetectorprovided on the fixed structure in register with the light beam; ashutter, coupled to the stylus-carrying member for movementcorresponding to movement of the stylus-carrying member, and positionedbetween the light source and the photodetector, the shutter having anaperture which permits the passage of a fraction of said light beam ontothe surface of the photodetector; wherein

said photodetector emits at least one signal which is indicative of thedisplacement in a given direction of the position of incidence of saidfraction of said light beam on said detector.

Embodiments of the present invention will now be described, by way ofexample, and with reference to the accompanying drawings in which:

FIG. 1 is a cross-section through a first embodiment of a probeaccording to the present invention;

FIG. 2 is a detail of FIG. 1;

FIG. 3 is a cross-section on the line III--III of FIG. 1;

FIGS. 4a and 4b are a further detail of FIGS. 1 and 3;

FIG. 5 is a cross-section through a second embodiment of a probeaccording to the present invention;

FIG. 6 is a detail of FIG. 5;

FIG. 7 is a cross-section of a modification to the probe of FIG. 5; and

FIG. 8 is a cross-section of a third embodiment of a probe according tothe present invention.

Referring now to FIG. 1, a measuring probe has a fixed structureprovided by a cylindrical housing 10 relative to which a stylusretaining member 12 is supported by a mechanism permittingthree-dimensional movement thereof relative to the housing 10. Thesuspension mechanism is carried inside a sleeve 14 which is connected tothe housing 10 via three bolts 16 which project through three flanges 18at the base of the sleeve 14, and form a screw-threaded engagement withan outwardly depending annular flange 20 at the base of the housing 10.The suspension mechanism comprises a cylindrical intermediate member 22supported by two resilient diaphragms 24A,B which are also connected tothe sleeve 14. The intermediate member 22 includes an inner hollowcylindrical member 26, defining a bore 28 through its center, and anouter cylindrical member 30. The diaphragms 24A,B are connected to theintermediate member 22 by a locking nut 32, having a screw-threadedengagement with the bore 28. The nut 32 bears against a spacer 34 tocompress the upper diaphragm 24A between the spacer 34 and the outermember 30, and the lower diaphragm 24B between the outer cylindricalmember 30 and a further spacer 36, which in turn bears against anoutwardly depending annular flange 38 provided at the base of the innermember 26. The intermediate member 22 is thus supported on thediaphragms for movement along an axis A relative to the housing 10; thediaphragms 24A,B however restrict any rotational movement about any axisperpendicular to the axis A.

Desirably, the diaphragms 24A,B should have a relatively lowforce-displacement characteristic in the direction of movement of theintermediate member along the axis A. One example of a design ofdiaphragm which achieves this is shown in FIG. 2 and comprises acircular sheet of material 40 in which are formed three slits 42,44,46,to leave essentially a three-legged core through which the intermediatemember 22 extends. Three legs denoted P,Q,R are connected respectivelyto a continuous annular periphery 48 by three tangential struts D,E, andF. While the diaphragms 24A,B constrain any lateral movements orrotations of the intermediate member 22 in the plane of the diaphragm,displacement of the intermediate member along the Z axis will result insome rotation of the intermediate member 22 relative to the housing 10.

Referring again to FIG. 1, the stylus carrying member 12 is supportedrelative to the intermediate member on a further diaphragm 50 which isclamped between a lower locking nut 52 and the outwardly dependingflange 38. The further diaphragm 50 is preferably relatively stiff todeflections of the stylus carrying member 12 relative to theintermediate member 22 in the direction of the axis A. However, thefurther diaphragm 50 permits relatively easy movement of the carryingmember 12 relative to the intermediate member 22 in the sense of tiltingrelative to the axis A. To this end, the further diaphragm 50 ispreferably a simple circular flat diaphragm spring having no slits orother modifications.

The carrying member 12 supports three balls 54, which project from itslower surface. The balls 54 serve to locate a stylus-carrying plate 56on the member 12 by engagement of each of the balls 54 in the convergentsurfaces provided by adjacently situated pairs of further balls 58provided in the upper surface of the plate 56. The plate 56 is retainedin engagement with the member 12 by magnets 60,62 provided on the member12 and the plate 56 respectively. This form of kinematic engagementbetween the plate 56 and the member 12 serves several functions:firstly, it enables the plate 56 to be removed from the engagement withthe member 12, and subsequently replaced without the need to re-datumthe probe to determine the position of the sensing tip 64 of the stylus66 supported on the plate 56; and secondly this engagement provides auseful crash protection feature, in that when the lateral force on thestylus increases to such an extent that damage to the suspensionmechanism may occur, the plate 56 will simply fall away from engagementwith the member 12. The force required to achieve this is dependent uponthe force between the magnets 60 and 62 which are thus chosenaccordingly.

The carrying member 12 further comprises an elongate stem 70 whichextends along, and projects beyond the inside of the bore 28 of theintermediate member 22. The free end of the stem 70 thus projects beyondthe space enclosed by the diaphragms 24A,B. The stem 70 carries, at itsfree end, a transducer member in the form of a shutter 74 having anL-shaped cross-section (see also FIG. 3). The shutter 74 is supported ona block 76 by a bolt 78; the block 76 is connected to the carrying stem70 by screw-threaded engagement of a bolt 80 with the inside of thecarrying member 70. The transducer member 74 is fixedly connected to thestylus-carrying member 12 for tilting movement therewith relative to theaxis A, and longitudinal movement along the axis A. Movement of thetransducer member 74 relative to the housing 10 is thus exactlyproportional to the corresponding movement of the sensing tip 64 of thestylus 66 relative to the housing 10.

The magnitude of movement of the transducer member 74 in directionsillustrated in FIG. 1 and X,Y,Z is detected by three sensors which areretained inside a transducer housing 82. Each sensor includes an LED 84and associated collimating lens 86 which together generate a beam oflight. A position-sensitive photodetector 88 is provided in registerwith this beam of light in the opposite side of the housing 82. Thetransducer member 74 comprises three apertures in the form of threeelongate slits 90A,B,C, which lie in register with the light beam of theassociated sensor. The slits 90A,B,C permit the passage of a fraction ofthe beam from the diode 84 onto the surface of the position-sensitivedetector 88; tilting of the stylus 64, and thus of the transducer member74, in, for example, the Y direction causes the slit 90B (see FIGS. 4aand 4b) to move relative to the position-sensitive detector 88. The slit90B remains within the light beam during this tilting movement, howeverthe displacement on the surface of the detector 88 in the Y direction ofthe fraction of light which passes through the slit 90B has changed byvirtue of movement of the slit. The output of the position-sensitivedetector 88 will thus change correspondingly, indicating a displacementof the stylus tip 64 in the Y direction. Identical transducingarrangements for movement of the transducer member 74 in the X and Zdirections are provided by the slits 90A,C and the associated emittersand detectors.

In order to position each slit 90A,B,C such that the fraction of lightincident upon the associated detector is in the middle of the detector'srange when the stylus-carrying member 12 lies at its rest position withrespect to the housing, the transducer housing 82 is adjustable in the Xand Y directions relative to the housing 10 by four adjusting screws 92.Adjustment of the height of the transducing member 74 with respect tothe transducer housing 82 is provided by a number of shims 94 situatedbetween the carrying block 76 and the upper end of the carrying stem 70.

The extent of tilting movement of the stylus-carrying member relative tothe housing 10 is limited by the clearance between the carrying stem 70and the bore 28. Thus, the lock nut 32 provides a stop for tiltingmovement of the carrying stem 70 in the X and Y directions relative tothe housing 10. A flange 98 provided on the supporting block 76 ismovable in the Z direction between limits defined by inwardly dependingflange 100 on the sleeve 14, and a lower lip 102 in the transducerhousing 82.

With this construction of the probe, the spring rate in the sense oftilting of the carrying member 12 relative to the axis A in the X and Ydirections is dependent completely upon the spring rate of the furtherdiaphragm 50 and the length of the stylus 66, whereas the spring rate inthe sense of displacements of the stylus-carrying member 12 in alongitudinal direction along the direction of the axis A (i.e. the Zdirection) is determined exclusively by the combined spring rate of thediaphragms 24A,B. Thus, the spring rate in respect of tilting of thestylus-carrying member 12 is completely independent of the spring ratein respect of longitudinal displacement of the carrying member 12. For agiven length of stylus therefore, it is possible to choose the springrate of the diaphragms 24A,B such that the force on the sensing tip 64will be equal for deflections in the X,Y and Z directions. Nota BeneWhile the actual spring rate of the further diaphragm 50 does not changewhen the length of the stylus 66 changes, a change in the length of thestylus changes the moment on the diaphragm 50 and thus the forcerequired to produce a given deflection of the stylus tip 64; this isthus equivalent to a change of the spring rate.

A further important aspect of the present invention lies in therelationship between the sensors in the transducer housing 82 and thetransducer member 74. The sensors in respect of each aperture 90A,B,Care stationary with respect to the housing 10. Each sensor thustransduces movement of the transducer member 74 in the respectivedirection relative directly to the housing. Cumulative errors due tomounting of sensors on moving parts of the probe is thus avoided. Also,because each of the slits 90A,B,C extends transversely to the directionin which the associated transducer measures, displacement of thetransducer member in a direction perpendicular to the measuringdirection is permitted without any loss of signal.

A second embodiment of the invention will now be described withreference to FIG. 5, in which a probe has an external housing 110,within which is mounted a cylindrical stylus assembly 112, having alongitudinal axis 112A, and which includes a stylus 114 connected to acylindrical body 115 by means of a screw-threaded connection 116. Thestylus 114 projects from the housing 110 and terminates in awork-contacting ball tip 118.

The support for the stylus assembly is in two parts. A first resilientsupport, in this example a resilient planar spring diaphragm 120, isclamped at its periphery to the housing 110 by means of clamping rings121 and 122 which are screwed together by screws 123. One end of thecylindrical body 115 is connected to the planar spring at its center bya further clamping arrangement, including an externally screw-threadedring 125 having a shoulder 126 and which is bonded, for example by glue,to the body 115. The diaphragm 120 has a central aperture 127 whichallows the body 115 to pass therethrough, but which is of lesserdiameter than the shoulder so that the spring rests on the shoulder. Asecond, clamping ring 128, which is internally screw-threaded, isscrewed onto ring 125 to clamp the spring onto the shoulder 126.

This first support allows tilting movement of the stylus assembly 112,and hence the stylus, about the plane of the diaphragm 120 when thestylus tip 118 is acted upon by forces in the X and Y axes directions.Additionally the first support allows movement of the stylus assembly112 linearly in the direction of the longitudinal axis 112A of theassembly when the stylus is acted upon by a force in the Z direction.However, the design of the diaphragm 120 is such that lateral movementsof the stylus assembly in the plane of the diaphragm are constrained, asare rotations about the Z axis while the diaphragm 120 remains in itsplane.

The second support which is introduced in accordance with the inventionis shown, in this example, to be a second resilient planar diaphragm130. The diaphragm 130 is clamped to the housing 110 between clampingrings 131 and 132 by screws 133. Ring 131 forms part of an end cap 134of the probe, while ring 132 is itself bonded to the housing, or screwedthereto by screws not shown.

The free end 119 of the stylus assembly (i.e. the end opposite to thestylus 114) is connected to the center of the diaphragm by means whichallows for transverse movement of said end, which in this example is awire 135.

The wire is formed as a reduced diameter portion of a rod which isfitted at one of its ends into the free end 119 of the cylindrical body115. The rod thus forms a plug 115A which is used as part of an opticaltransducing system (see FIG. 6).

At its other end the rod is connected to the central portion of thediaphragm 130 by a clamping arrangement which includes a shoulder 136 onthe rod and a spacer 137 which passes through the end cap 134. Thediaphragm 130 is clamped between the shoulder 136 and the spacer 137 bya screw 138 and washer 139. The arrangement is such that although thediaphragm 130 is tightly clamped by the spacer, the spacer provides aclearance 140 between the end cap and the washer 139. The clearance 140allows for the axial movement of the stylus assembly, but the washer 139provides an end stop to limit the axial movement to preventoverstraining of the resilient supports in the event of an excessiveforce being applied to the stylus 114.

The diaphragm 130 will provide resistance to movement of the stylus inthe Z direction in addition to that provided by the first diaphragm 120.Similarly the combination of the wire 135 and the second diaphragm 130will provide resistance to deflection of the stylus in the X and Ydirections which is in addition to that provided by the first diaphragm120. Thus for at least one stylus length, it is possible to design thecombination of the two diaphragms 120 and 130 and the wire 135, toensure that the same deflection of the stylus tip 118 in the X, Y or Zdirections will result from the application of a given forceirrespective of the direction in which the force is applied to thestylus.

The design of the diaphragms is as illustrated in FIG. 2. An alternativediaphragm design which allows for X, Y tilting and Z axis displacementof the stylus assembly but which prevents rotation of the stylus aboutits axis is shown in UK Patent Specification No. 1,573,477.

FIG. 6 shows the detail of the transducing system. Apertures 145,146 and147 are made through the plug 115A, which is the part of the stylusassembly furthest from the planar spring 120 to enable the maximum X andY deflections to be sensed. Light sources 148,149 and 150 (preferablyLED) are provided on the housing 110 at the same height as theapertures, so that, in operation, the light beams from the sources passthrough the apertures to fall onto split photo-diode detectors 151,152and 153 on the housing opposite to the LEDs. The detectors produceelectrical signals in dependence upon the amount of light falling oneach half of the detector, and by using the arrangement described below,the signals can be made to indicate the magnitude of the X, Y and Zdeflections of the stylus assembly.

The three detectors have their splits oriented so that only one detectorwill detect deflection of the stylus in any of the three directions X, Yand Z. Thus, for example, LED 148 shines through an aperture 145 onto asplit detector 151 which has a split lying in the X,Y plane and detectsprimarily Z axis movements of the stylus assembly. Similarly LED 149projects a beam through an aperture 147 onto a split detector 152 whichhas a split lying in the X,Z plane and detects primarily Y axismovements of the stylus assembly, while LED 150 shines through anaperture 146 in the stylus assembly, which lies at right angles toapertures 145 and 147, onto a split detector 153 which has a split lyingin the Y,Z plane, and detects primarily X axis movements of the stylusassembly.

As with the previous embodiment, the tilting of the stylus will giverise to second order movements of the apertures in the stylus in the Zdirection in addition to the X, or Y movements but these are so small asto be negligible.

In an alternative embodiment of the invention the second diaphragm 130and wire 135 are replaced by three or more additional coil springs,which extend transversely between the free end of the stylus assembly115 and the housing 110 may be provided in place of the second diaphragm130 and wire 135.

Thus, when the stylus 114 is acted upon by forces in the X and Ydirections and tilts about diaphragm 120, one or more of the coilsprings will undergo a change in length which will provide an additionalresistance to such tilting motion. However, when the stylus is actedupon by a purely axial force in the Z direction the springs will bendand/or pivot about their anchor points and will add a significantlysmaller resistance to axial deflection of the stylus.

Hence, for example, by careful design of the combination of thediaphragm 120 and the coil springs, the deflection of the stylus tip inthe X, Y or Z directions for any given combination of forces can be morenearly equalised.

In the embodiment of FIG. 5 the stylus has been described as beingscrewed into the stylus assembly, but in an alternative design it may bemagnetically clamped to the remainder of the stylus assembly so that itmay be easily and quickly changed for another stylus suitable for adifferent scanning operation. Also, as mentioned earlier, such amagnetic clamping arrangement provides crash protection, preventingdamage to the diaphragms in the event of excessive force on the stylus.

An example of such a construction is shown in FIG. 7. Parts similar tothose illustrated in FIG. 5 are given the same reference numerals. Thebody 115 is provided with three equally spaced, radially extending ribs160 at its end, and the stylus 114 is provided with similar ribs 161 atits end. Each of the ribs 160 is provided with a cylindrical roller 162at the radially outer extremity of its end surface, and each of the ribs161 is provided with a pair of balls 163 at the radially outer extremityof its end surface. The balls and rollers provide a kinematic seatingarrangement for the stylus on the body 115.

A pair of confronting magnets 165,166 are provided respectively on thebody 115 and the stylus 114 which hold the stylus in position undernormal applications of force to the stylus. When an excessive tiltingforce is applied to the stylus the magnetic connection will breakallowing the stylus to fall off before any damage can occur to theresilient supports.

A further embodiment of probe will now be described with reference toFIG. 8. The probe comprises a fixed structure in the form of acylindrical housing 210, by which the probe may be supported on themovable arm of a coordinate positioning machine on which it is to beused. A stylus carrying member 212 is supported on a first diaphragm214, and is connected, via screw-threaded engagement 216 to a stylus 218having a spherical sensing tip 220. The diaphragm 214 is clamped to thecylindrical housing 210 between two clamping rings 222,224 which areurged together by bolts (not shown). The diaphragm 214 permits tiltingmovements of the stylus-carrying member 212 and stylus 218 in the X andY directions, together with longitudinal movement of the stylus-carryingmember 212 in the Z direction. The stylus-carrying member 212 iscoupled, via a short rod spring 226 to a transmission member 228, whichis in turn supported on a further diaphragm 230. The further diaphragm230 is rigidly clamped to the housing 210 between clamping rings232,234, which are urged together by bolts (not shown). The furtherdiaphragm 230 permits tilting movement of the transmission member 228 inthe X and Y directions and longitudinal movement of the member 228 inthe Z direction.

The transducing system for measuring the magnitude and direction ofdeflections of the stylus tip 220 from a nominal rest position isidentical to the system used in the first embodiment of the presentapplication. Briefly, the system comprises an L-sectioned transducermember 236 supported directly on the transmission member 228, andextending above the further diaphragm 230. As before, the transducermember 236 includes three slits, two of which are illustrated here underthe designations 238A,238C. Sensors associated with each slit comprisean LED 240 and associated collimating lens 242 which generate a beam oflight; a position-sensitive photodetector 244, also supported on thehousing is positioned in register with the beam of light. Movement ofthe transducer member causes movement of the slits within the boundariesof the respective beams, causing corresponding movement of the fractionof the beam transmitted by the slit across the surface of thephotodetector 244, which thus results in a change in the output of thephotodetector indicative of the position of transducer member 236 andthus the stylus tip 220.

The rod spring 226 serves to provide additional resistance to tiltingmovement in the X and Y directions of the stylus-carrying member 212,thereby increasing the force required to tilt the carrying member 212and thus the stylus 218. By selecting the stiffness of the rod spring226 in accordance with a predetermined given length of stylus 218, theforces required to deflect the stylus 218 in the X,Y and Z directionscan be equalised. The rod spring 226 should be as short as possible sothat the deflection of the stylus tip 220 always corresponds todeflections of the transducer member 236 at high frequencies ofmovement; this ensures that the movement of the transducer memberremains in phase with movement of the stylus.

Probes made in accordance with the invention as described above have arelatively small range of stylus deflections. These can be accuratelyrecorded by the optical sensing system and thus the probe lends itselfto use with a fast acting servo system (e.g. one with an update periodat least as short as 1 millisecond) to produce a scanning system whichis fast, accurate and relatively inexpensive.

In a modification of the probes shown in FIGS. 1 and 5, the pairs ofdiaphragms 24A,B and 120,130 may be used mainly for stylus guidance,with a separate axial spring being provided to supply the greater partof the axial force for restoring the stylus to its rest position. Thisis another method of adjusting the spring rate in the axial (Z)direction.

The relative spring rates for deflections of the stylus assembly in theZ, and X,Y directions are chosen in accordance with inter alia thealgorithm used to control movement of the machine on which the probe isemployed. It can be seen in the embodiments described in FIGS. 1, 5 and8, that no separate axial spring is used. In these examples if the axialrestoring force provided by the diaphragms is relatively small withinthe permitted range of movement of the stylus in the Z direction thenwith a lightweight hollow stylus assembly this will give a fast dynamicresponse to the probe in the vertical Z direction, and a much slower onein the X and Y directions. This design reduces the problems caused bythe stylus sliding sideways around obstructions on a workpiece surface.In such a probe, although the symmetry of force versus stylusdisplacement is not achieved, the faster Z response can be used incombination with a fast-acting servo control system to minimise theearlier described problem. Alternatively, the diaphragms providing axialmovement may be chosen so that their spring rate is relatively high,thus producing a probe in which the displacement characteristics aresymmetrical.

Sensors other than optical sensors as described may be used providedthat they can operate at sufficient speed. Thus for example, inductive,or capacitance sensors may be used to sense the X, Y or Z deflections.Also strain gauges may be positioned on the diaphragms or the stylus todetermine the three components of the deflection of the stylus tip.

Where optical sensors are used the position-sensitive detector of FIGS.1 to 4, or the split detector of FIGS. 5 and 6 may be replaced by acharge coupled device array and an interpolator, or other suitablephotodetector.

We claim:
 1. A measuring probe for use on a coordinate positioningapparatus, the probe comprising: a fixed structure by which the probemay be supported on a movable arm of the machine, and a stylus retainingmember for retaining a workpiece-contacting stylus supported relative tothe fixed structure by a suspension mechanism permitting at least twodimensional movement of the retaining member relative to the fixedstructure, the probe further comprising transducer means for measuringdisplacement of the stylus retaining member in three perpendiculardirections relative to the fixed structure, the transducer meanscomprising:three light sources for generating three beams of lightextending in three different directions; three photodetectors retainedon the fixed structure, each photodetector lying in register with one ofthe light beams; a shutter coupled to the stylus retaining member formovement relative to the fixed structure which corresponds to movementof the stylus retaining member relative to the fixed structure, saidshutter located in the path of each of said three light beams andincluding three apertures each of which permits the passage of afraction of one of said light beams onto the surface of thecorresponding photodetector; each of said three photodetectors emittingat least one signal which is indicative of the displacement in a givendirection of the position of incidence of said fraction of saidrespective light beam on said detector; wherein: said three aperturesare provided by three mutually perpendicular slits each of which a)extends in a direction transverse to said given direction and b) definesa direction of movement of said retaining member to which thecorresponding photodetector is insensitive.
 2. A measuring probeaccording to claim 1 wherein said shutter is mounted rigidly to saidstylus retaining member.
 3. A measuring probe according to claim 1wherein each of said light sources is provided on the fixed structure.4. A measuring probe according to claim 1 further comprising collimatingmeans for collimating each of said light beams.
 5. A measuring probeaccording to claim 1 wherein said shutter has an L-shaped cross-section.6. A measuring probe according to claim 1 wherein each of said lightsources and said photodetectors are provided in a transducer housingextending around said shutter, and wherein the position of thetransducer housing is adjustable relative to the fixed structure toenable alignment of each aperture with the respective light beams.
 7. Ameasuring probe according to claim 6 wherein said suspension mechanismenables tilting movements of said retaining member relative to the fixedstructure.
 8. A probe for use on a coordinate positioning apparatus, theprobe comprising: a fixed structure by which the probe may be supportedon a movable arm of the machine, and a stylus retaining member forretaining a workpiece-contacting stylus supported relative to the fixedstructure by a suspension mechanism permitting at least two dimensionalmovement of the retaining member relative to the fixed structure, theprobe further comprising a transducer assembly that measuresdisplacement of the stylus retaining member relative to the fixedstructure from a null position, the assembly comprising:a light sourcefor generating a beam of light; a photodetector retained on the fixedstructure and lying in register with the beam of light; a shuttercoupled to the stylus retaining member for movement relative to thefixed structure which corresponds to movement of the stylus retainingmember relative to the fixed structure, said shutter being positionedbetween the light source and the photodetector, said shutter includingan elongated slit aperture that permits the passage of a fraction ofsaid light beam onto the surface of the photodetector,said elongatedslit aperture extending in a first direction; said photodetectoremitting at least one signal that is indicative of displacement in asecond direction transverse to said first direction of a position ofincidence of said fraction of said light beam passing through saidelongated slit onto said photodetector.
 9. The probe according to claim8, wherein said shutter is mounted rigidly to said stylus retainingmember.
 10. The probe according to claim 8, wherein said light source isprovided on said fixed structure.
 11. The probe according to claim 8,further comprising a lens that collimates said light beam.
 12. The probeaccording to claim 8, wherein said light source and said photodetectorare provided in a transducer housing extending around said shutter, andwherein the position of the transducer housing is adjustable relative tothe fixed structure to enable alignment of the aperture with the lightbeam.
 13. The probe according to claim 8, wherein said suspensionmechanism enables tilting movement of said retaining member relative tothe fixed structure.